I've found images are really, really important to folks and they often see them as integral to the conversation rather than as standalone gallery items per se. It takes some forethought and extra steps to think, "well first I'll post this into to a gallery album so I can reference it and then go back to the chit-chat forum and embed the image that goes with the story I want to tell so we can talk about it."
I suspect the difference may be the subtle difference between a comment and a reply. You can certainly have a discussion in comments, but forums are set up and people are conditioned to use them as the meeting place, while galleries tend to have more of a museum or portfolio quality. In the gallery people tend to interact with the image and the poster, while in the forum people interact with everyone else in addition to the image and the poster.
I would like to see increased visibility across image sources, although I don't personally see the need to specifically integrate attachments into the gallery (although that's certainly one way to accomplish it). I would be happy to simply see per-user image results aggregated in a "show content" view from their profile, honoring whatever permissions exist for wherever the content came from.